Various biosynthetic compounds are produced in the natural metabolic process and used in different industrial fields such as food, feedstuff, cosmetic, and pharmaceuticals. The compounds have been produced by using bacterium or other microorganism developed for producing and secreting them in a large scale. For examples, Corynebacterium species has been used in the industry of amino acid production. In middle of 1950, Corynebacterium glutamicum producing glutamic acid efficiently and, auxotrophic mutant of Corynebacterium glutamicum has produced various amino acids by using fermentation.
The expression of various related genes can be regulated accurately for cell engineering therefore requiring the efficient expression system. Different components of cell regulating sequence have been known in the art. The examples of the components are a binding region to the operator, a RNA polymerase binding region of −35 and −10 regions, and a ribosome binding site or Shine-Dalgarno sequence in ribosomal 16S RNA.
It is important to select a promoter to develop the expression system, because the promoter is largely involved in the gene expression level and expression regulation. Several promoters being applicable to Corynebacterium glutamicum have been reported and are derived from Corynebacterium sp. or E. coli (J. Biotechnol., 104:311-323, 2003).
However, the promoter derived from E. coli a low permeability of an expression inducer and absence of gene expression inhibitor and thus shows low activity relative to that of Corynebacterium. Even if the same promoters are used, their activities are different depending on the coating sequence of target gene. The promoters used in Corynebacterium have a difficulty in being prepared for the desired object, because it is a narrow of choice in the expression level of promoters. Especially, when the expressions of various genes are regulated together such as the establishment of metabolic pathway, Corynebacterium, various promoters cannot be selected, unlike E. coli. 
Psicose is getting a spotlight in a diet sweetener, but is required to be produced in a large scale for applying to the food due to rare sugar in nature. In the prior art, Psicose has been largely produced by synthetic chemical method. As the enzymatic method, KR10-0744479 discloses the mass production of psicose using the psicose epimerase produced by E. coli transformed with the coding gene of psicose epimerase derived from Agrobacterium tumefaciens. There is a method of producing psicose by using a microorganism producing enzyme without purification in a low production cost. In the disclosure of KR10-1106253, the recombinant E. coli which is transformed with the coding gene of psicose-3-epimerase derived from Agrobacterium tumefaciens and includes an inactivated specific gene, is inoculated on the culture medium including fructose to convert the fructose to psicose.
The recombinant E. coli used in KR10-1106253 is not GRAS (Generally Recognized As Safe) microorganism, and thus cannot be suitable in food industry. In addition, the Psicose epimerase derived from Agrobacterium tumefaciens has a low enzyme activity and heat stability.
Therefore, there is a need to develop an expressing being capable of producing the Psicose epimerase having a high enzyme activity in GRAS microorganism at a high yield and stably expression system, a method of psicose epimerase using the expression system, and a method of psicose by using the enzyme or transformed GRAS microorganism.